The present invention relates to an electrochromic device having improved switching behavior.
Electrochromic devices are already known, for example, from D. Theis in Ullmann""s Encyclopaedia of Industrial Chemistry, Vol. A 8, p. 622, Verlag Chemie 1987 and WO-A 94/23333. A distinction is made between two basic types:
Type 1: full-area electrochromic devices; and
Type 2: electrochromic display devices having structured electrodes.
Type 1 is used, for example, in electrically darkenable window panes or electrically dimmable automobile mirrors. Such devices are disclosed, for example, in U.S. Pat. No. 4,902,108. Type 2 is used in segment and matrix displays. Such display devices are proposed, for example, in DE-A 196 31 728. Devices of this type can be observed transmissively or, in the case of reflection, reflectively.
WO-A 94/23333 compares electrochromic materials having different constructions, but these are not used as display devices:
Construction a: the electrochromic substances are in the form of a fixed film or layer on the electrodes (cf. Ullmann, see above).
Construction b: the electrochromic substances are deposited on the electrodes as a layer by the redox process (cf. Ullmann, see above).
Construction c: the electrochromic substances remain permanently in solution.
For construction a), the best-known electrochromic material is the tungsten oxide/palladium hydride pair. For construction b), viologens have been described as electrochromic substances. These devices are not self-erasing, i.e., the image produced remains after the current has been switched off and can only be erased again by reversing the voltage. Such devices are not particularly stable and do not allow a large number of switching cycles.
In addition, the cells constructed using tungsten oxide/palladium hydride in particular cannot be operated in transmitted light, but only reflectively, owing to light scattering at these electrochromic layers.
Elektrokhimiya 13, 32-37 (1977), U.S. Pat. No. 4,902,108 and U.S. Pat. No. 5,140,455 disclose an electrochromic system of the latter construction c). An electrochromic cell built up from glass plates with a conductive coating contains a solution of a pair of electrochromic substances in an inert solvent.
The pair of electrochromic substances used is one electrochemically reversibly reducible substance and one reversibly oxidizable substance. Both substances are colorless or only weakly colored in the ground state. Under the action of an electric voltage, one substance is reduced and the other oxidized, both becoming colored. When the voltage is switched off, the ground state re-forms in the case of both substances, decolorization or a color lightening taking place. 
U.S. Pat. No. 4,902,108 discloses that suitable pairs of redox substances are those in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves.
Various applications have been described for such electrochromic cells of construction c. For example, they can be formed as automobile rear-view mirrors which can be darkened during night driving by application of a voltage and thus prevent dazzling by the headlamps of following vehicles (cf. for example, U.S. Pat. Nos. 3,280,701, 4,902,108 and EP-A 0 435 689). Furthermore, such cells can also be used in window panes or automobile sunroofs, where they darken the sunlight after application of a voltage. Likewise described is the use of such devices as electrochromic display devices, for example, in segment or matrix displays having structured electrodes (DE-A 196 31 728).
The electrochromic cells normally consist of a pair of glass plates, of which, in the case of the automobile mirror, one is mirrored. One side of these plates is coated over the full area with a light-transparent, electroconductive layer, for example indium-tin oxide (ITO), where, in the case of display devices, this conductive coating is divided into electrically separated segments provided with individual contacts. These plates are used to construct a cell by bonding them by means of a sealing ring with their electroconductively coated sides facing one another to form a cell. This cell is filled with an electrochromic liquid via an opening, and the cell is tightly sealed. The two plates are connected to a voltage source via the ITO layers.
Electrochromic devices of construction c, as described in greater detail above, are self-extinguishing, i.e., the coloring of the device disappears of its own accord when the voltage is switched off. This can be accelerated, for example, by short-circuiting the poles of the device or by brief application of a voltage of opposite polarity. In accordance with the above-mentioned equation, the uncolored molecules RED1 and OX2 then re-form through electron transfer from the colored molecules OX1 and RED2.
However, this self-extinguishing may be considerably slowed, in particular in the case of display devices in which the electrically conductive coating on at least one of the two plates or films is divided into individually contacted segments, in particular if voltage was applied to individual segments for an extended period.
Surprisingly, it has now been found that the self-extinguishing of an electrochromic device, particularly an electrochromic display device, can be considerably accelerated if the electrochromic medium, in addition to the electrochromic substances OX2 and RED1, also contains a further reducible or oxidizable electrochromic compound OX3 or RED3 respectively whose reduction or oxidation potential respectively has a value which is not greater than the reduction potential of OX2 or the oxidation potential of RED1.
It has been found that a further advantage of an electrochromic device of this type which contains an OX3 or RED3 in addition to OX2 and RED1 is that the edge sharpness of the switched segments of an electrochromic display device is significantly increased.
The invention relates to an electrochromic device that comprises a pair of glass plates, a pair of plastic plates, or a pair of plastic films, and an electrically conductive coating located on a side of at least one of the glass plates, the plastic plates, or the plastic films. The pair of glass plates, plastic plates or plastic films are joined together via a sealing ring located on the side in which the electrically conductive coating is located such that the pair of glass plates, plastic plates, or plastic films and the sealing ring form a volume. The term xe2x80x9cvolumexe2x80x9d herein is interchangeable with the term cavity.
The invention provides an electrochromic medium that is contained in the volume that comprises (i) a first reducible electrochromic compound and a second reducible electrochromic compound having a reduction potential that is less than the reduction potential of the first reducible electrochromic compound, and (ii) a first oxidizable compound and a second oxidizable compound having an oxidation potential that is less than the oxidation potential of the first oxidizable compound in which at least one plate or at least one film and the conductive coating is transparent, such that the electrically conductive coating in at least one of the plastic plates, the glass plates, or the two films can be divided into separate, individually contacted area segments and the conductive coatings can be connected at at least one point to an electrical conductor. The invention is also directed to an electrochromic medium.